High voltage applications, particularly electrically powered and hybrid (conventional fossil fuel power in combination with electrical power) vehicles require relatively large capacity battery system to deliver a relatively large amount of power compared to a 12 volt automobile storage battery. Since power is directly proportional to battery voltage and system current, the high power delivery requirements which must be satisfied by such batteries mean that higher electrical voltages will be present in electric and hybrid vehicles than in conventionally powered vehicles. Such vehicles are becoming increasingly attractive alternatives to fossil fuel powered cars. However, because of the high voltage requirements of its traction battery an electric or hybrid electric vehicle (HEV) raises significant electrical safety concerns.
For example, unwanted electric current flow outside of the intended circuit path (i.e. dielectric breakdown faults, ground fault conditions and the like) may cause significant damage to electronic components within a system (such as an electric vehicle or hybrid electric vehicle propulsion system), thereby disabling or even destroying the electronic equipment. In addition, such ground fault conditions may result in an electric shock, which can have more serious safety consequences when the shock is caused by contact with a high voltage battery system, as compared to a conventional, relatively low voltage automotive storage battery system. To reduce the likelihood of such shock, many high voltage battery systems are not grounded to the frame of the machine or vehicle chassis.
Instead, high voltage battery systems have a closed loop return path, so that the negative power conductor of the system (i.e., the electrical current return path) is isolated from the frame or chassis of the machine, electric vehicle or HEV.
While such isolated systems may minimize the likelihood of a significant electric shock to a person in the event of a short circuit or low impedance connection (i.e. dielectric breakdown fault), certain electronic components typically in electrical communication with the positive and negative power conductors (bus lines or rails) that supply high voltage power are subject to damage resulting from extreme voltage or current swings occurring thereon.
Existing high voltage standards relating to ground fault detection, including Federal Motor Vehicle Safety Standard (FMVSS) 305, require a minimum response time for detection under constant monitoring of the isolation parameters in both DC and AC circuits. In addition, these standards require detection of an isolation fault within 100 milliseconds and report of any such fault within 50 milliseconds of detection. The minimum isolation resistance recommended by the SAE is 500 ohms per volt and it is commonly preferred to set this measurement to at least twice the SAE minimum or 1000 ohms per volt.
Typically previously known fault detection circuits typically use resistor/capacitor networks requiring multiple measurement circuits to provide detection of dielectric breakdown resistance. This configuration results in greater expense due to the multiple measurement circuits required and slower than desired detection times due to the time constant created by the resistor/capacitor network. In addition, these circuits must reach steady state to obtain an accurate measurement which is an undesirable operational limitation. Furthermore these known detection circuits must pulse or switch high voltage to the chassis during measurement causing additional noise to be created in addition to the dangers associated with such a high voltage pulse. Moreover, prior art systems are not capable of measuring the ground fault resistance in both DC and AC circuits which provides an advantage in circuit operation, reducing circuit construction costs and meeting the standards of ground fault detection noted above.
Such a prior art fault detection circuit is shown in FIG. 1 and is indicated generally by reference number 10. Prior art circuit 10 includes an isolated high voltage battery 12 with voltage V pack. As shown in FIG. 1, a leakage path is depicted by reference numeral 16 through resistance Rleg 18. Typically, battery 12 is grounded along with the line 20 to the vehicle chassis 22 through capacitors Cy 24, 26. As noted above, prior art system 10 must reach a steady state wherein no current is flowing through capacitors Cy 24, 26 to provide an accurate measurement of the dielectric breakdown resistance. To achieve this steady state condition, both R-C loops 28, 30 must be in a steady state condition before taking the Va and Vb readings necessary to calculate the dielectric breakdown resistance Rleg. Such a steady state requirement introduces less than desirable response time in detecting a dielectric breakdown fault. In addition, this prior art detection circuit must charge and discharge Vpack through the chassis of the vehicle which creates the potential for noise and electric shock through the chassis. Furthermore, this circuit varies and is dependent on the capacitance of the circuit which creates difficulty is accurately detecting and measuring the resistance of the dielectric breakdown fault. Importantly, this prior art circuit does not meet the detection time requirement of 100 ms as noted above in the Federal Motor Vehicle Safety Standard (FMVSS) 305 specification.
Accordingly, it is an object of the present invention to provide a system and method for detecting faults in high voltage battery systems which provide quick, accurate and cost effective fault detection in both DC and AC circuits, is safe and which does not unduly cause system battery drain. Another object of the present invention is to provide a system and method for detecting faults in high voltage, electric vehicle and hybrid vehicle battery systems which measures the dielectric breakdown system (DBS) resistance and detects the DBS fault to the chassis or frame when the DBS resistance is 35,000 ohms or less. Further, it is an object of the present invention to provide a system and method for detecting faults in high voltage, electric vehicle and hybrid vehicle battery circuits which detects the DBS fault to the chassis or frame and measures the dielectric breakdown system (DBS) resistance which is independent of the capacitance of the circuit. Still another object of the present invention is to provide a system for detecting faults in high voltage battery systems which is simple in construction, quick in detection response, does not introduce external current into the circuit to obtain a measurement, is easy to use and is cost effective.